feat: download the latest version instead of a pinned one

README.md

@@ -46,9 +46,16 @@ Just use your package managers as you usually would. Run `yarn install` in Yarn
 
 ## Known Good Releases
 
-When running Yarn or pnpm within projects that don't list a supported package manager, Corepack will default to a set of Known Good Releases. In a way, you can compare this to Node.js, where each version ships with a specific version of npm.
+When running Corepack within projects that don't list a supported package
+manager, it will default to a set of Known Good Releases. In a way, you can
+compare this to Node.js, where each version ships with a specific version of npm.
 
-The Known Good Releases can be updated system-wide using the `--activate` flag from the `corepack prepare` and `corepack hydrate` commands.
+If there is no Known Good Release for the requested package manager, Corepack
+looks up the npm registry for the latest available version and cache it for
+future use.
+
+The Known Good Releases can be updated system-wide using the `--activate` flag
+from the `corepack prepare` and `corepack hydrate` commands.
 
 ## Offline Workflow
 
@@ -106,6 +113,8 @@ This command will retrieve the given package manager from the specified archive
 
 - `COREPACK_ENABLE_NETWORK` can be set to `0` to prevent Corepack from accessing the network (in which case you'll be responsible for hydrating the package manager versions that will be required for the projects you'll run, using `corepack hydrate`).
 
+- `COREPACK_NO_LOOKUP` can be set in order to instruct Corepack not to lookup on the remote registry for the latest version of the selected package manager.
+
 - `COREPACK_HOME` can be set in order to define where Corepack should install the package managers. By default it is set to `$HOME/.node/corepack`.
 
 - `COREPACK_ROOT` has no functional impact on Corepack itself; it's automatically being set in your environment by Corepack when it shells out to the underlying package managers, so that they can feature-detect its presence (useful for commands like `yarn init`).

sources/Engine.ts

@@ -7,6 +7,7 @@ import defaultConfig                                          from '../config.js
 
 import * as corepackUtils                                     from './corepackUtils';
 import * as folderUtils                                       from './folderUtils';
+import {fetchAsJson}                                          from './httpUtils';
 import * as semverUtils                                       from './semverUtils';
 import {Config, Descriptor, Locator}                          from './types';
 import {SupportedPackageManagers, SupportedPackageManagerSet} from './types';
@@ -69,17 +70,26 @@ export class Engine {
       // Ignore errors; too bad
     }
 
-    if (typeof lastKnownGood !== `object` || lastKnownGood === null)
-      return definition.default;
+    if (typeof lastKnownGood === `object` && lastKnownGood !== null &&
+        Object.prototype.hasOwnProperty.call(lastKnownGood, packageManager)) {
+      const override = (lastKnownGood as any)[packageManager];
+      if (typeof override === `string`) {
+        return override;
+      }
+    }
 
-    if (!Object.prototype.hasOwnProperty.call(lastKnownGood, packageManager))
+    if (process.env.COREPACK_NO_LOOKUP)
       return definition.default;
 
-    const override = (lastKnownGood as any)[packageManager];
-    if (typeof override !== `string`)
-      return definition.default;
+    const {[`dist-tags`]: {latest}, versions: {[latest]: {dist: {shasum}}}} = await fetchAsJson(`https://registry.npmjs.org/${packageManager}`);
+    const reference = `${latest}+sha1.${shasum}`;
+
+    await this.activatePackageManager({
+      name: packageManager,
+      reference,
+    });
 
-    return override;
+    return reference;
   }
 
   async activatePackageManager(locator: Locator) {

tests/main.test.ts

@@ -6,6 +6,7 @@ import {runCli}                                    from './_runCli';
 
 beforeEach(async () => {
   process.env.COREPACK_HOME = npath.fromPortablePath(await xfs.mktempPromise());
+  process.env.COREPACK_NO_LOOKUP = `1`;
 });
 
 const testedPackageManagers: Array<[string, string]> = [